The proton exchange membrane fuel cell (PEMFC) continues to benefit from intense development efforts for its potential application in automobiles and distributed power generation due to a number of inherent advantages including high efficiency, low noise and chemical emissions, and low operating temperature. A PEMFC typically consists of a membrane electrode assembly (MEA), gas diffusion electrode (GDE) layers and bipolar plates. The MEA consists of an anode, a cathode and a membrane electrolyte and is the key element of the fuel cell. During PEMFC operation, hydrogen is electro-oxidized at the anode. The proton thus produced is transported through the electrolyte and is combined with an oxide ion formed through the reduction of oxygen at the cathode. At present, the electrode catalyst materials used at the anode and the cathode are primarily platinum supported over amorphous carbon. Since platinum is a precious metal with limited supply, reducing its usage will result in significant reduction in PEMFC cost for the commercialization. One of the contributing causes of high precious metal usage is inefficient utilization of the precious metal the present electrode catalyst preparation method. Generally, the MEA preparation steps involve catalyst synthesis, ionomer/catalyst ink preparation and casting catalyst ink onto the membrane electrolyte. The catalyst synthesis usually is accomplished through a wet chemical process in which the precious metal precursor is deposited over a high surface area carbon followed by a chemical reduction. The electrode catalyst thus prepared has highly dispersed metal crystallites distributed throughout the surface of carbon black. The catalyst is subsequently mixed with a polymer solution, known as ionomer, to form the ink. The ink is then cast over each side of the polymer electrolyte through a hot-pressing method to form the MEA. An intrinsic limitation to this approach is that a significant amount of catalyst is embedded underneath of the polymer matrix during the hot-pressing, rendering it inaccessible to gas flow. Therefore, these catalyst can not participate in the electro-chemical reaction are thus considered under utilized.
A GDE is another key component in a PEMFC. The GDE is typically made of carbon paper or cloth treated with a hydrophobic coating. A GDE is packed at each side of the MEA between electrodes and the bipolar plates to improve the electric conductivity, humidity control as well as reactant gas distribution. A GDE adds additional manufacturing cost and complexity to a PEMFC fabrication. The bipolar plate in PEMFC is made of corrosion resistant, electric conducting materials such as graphite or surface treated stainless steel. Complicated gas flow channels, known as the flow field, are often required to be embossed on the bipolar plate surface to distribute the hydrogen or oxygen uniformly over each side of the MEA. The bipolar plate also electrically connects the adjacent fuel cell modules to form the PEMFC stack. Construction of a flow field on a bipolar plate adds cost and complexity to the PEMFC fabrication process.
Wilson and Gottesfeld summarized the conventional method of preparing membrane electrode assemblies for a PEM fuel cell as disclosed in Wilson and Gottesfeld, Journal of Applied Electrochemistry 22, (1992) pp. 1-7, incorporated herein by reference, discloses the method of forming thin film catalyst layers for MEA by preparing ink containing amorphous carbon supported precious metal, followed by applying the ink and hot-pressing. Grot and Banerjee U.S. Pat. No. 5,330,860 incorporated herein by reference, further describes a method of preparing electro-catalyst ink and frication of MEAs with such ink. Harada U.S. Pat. No. 5,399,184 incorporated herein by reference, discloses a method of making MEAs and a fuel cell assembly with gas diffusion electrodes (GDE). Wilkinson et al. U.S. Pat. No. 5,521,018 and disclosed herein by reference, discloses a design of bipolar plate with embossed fluid flow field that has functions of conducting electricity and directing the reactant gas flow.
The inventive method is different from these conventional approaches because: (a) there is no need to prepare a carbon based catalyst ink through mixing before transferring the films to MEAs. Aligned carbon nanotube layers are transferred to a membrane electrolyte with the nanotube orientation and pattern remaining intact: (b) the aligned carbon nanotubes in the present invention have excellent electric conductivity and hydrophobicity, therefore rendering the application of GDE unnecessary; (c) a gas flow field pattern can be optionally built during the preparation of the aligned carbon nanotube bundles according to the invention; therefore, there is no need for embossing the flow field in bipolar plate thus minimizing the manufacturing cost.
A recent patent application by McElrath et al. U.S. Publication No. 2004/0197638 A1, incorporated herein by reference, discloses a method of preparing a membrane electrode using carbon nanotube materials including the steps of suspending nanotubes in solution, filtering nanotubes to form thin mat or dried catalyst ink over a membrane electrolyte. This invention is different in the following aspects: (a) carbon nanotubes are transferred directly to the membrane electrolyte without liquid suspension or filtration; and (b) carbon nanotube bundles are aligned in the same direction with optional 3-dimensional pattern whereas the carbon nanotubes in the prior art can not be aligned due to limitations in the method of preparation.
Another recent patent application by Toops, U.S. Publication No. US 2004/0224217 A1, disclosed herein by reference, discloses a method of preparing aligned carbon nanotube for MEA fabrication by pyrolyzing hydrocarbons inside of porous channels of an anodized alumina template, followed by dissolving the alumina with acid. This invention is superior in the following aspects: (a) the aligned carbon nanotubes in this invention are prepared through growth over a substrate plate through chemical vapor deposition without the need of an alumina template to guide the vertical alignment, obviating the cost of an alumina template and the acid removal process; and (b) this method can produce carbon nanotube electrodes with three dimensional patterns as the result of the preparation of substrate. After transferring on to the membrane electrolyte, a pattern can be formed in which certain nanotubes are lower in height than the neighboring ones. One of such patterns is the straight channel, as is shown in FIG. 6. These channels serve as the distribution conduits for the reactant gas flows uniformly throughout the electrodes, which have advantage of replacing flow field in bipolar plate in the conventional design. In addition, the electrode prepared with the aligned carbon nanotubes according to the invention has higher nanotube density than the prior art. Because the inventive method is template free, nanotubes can grow closely and in contact with each other, as is shown in FIG. 4. The prior art requiring alumina templates does not have this flexibility.